1. Field of the Invention
This invention relates to a preignition detecting system in an internal combustion engine and, more particularly, to a preignition detecting system capable of detecting a preigniting condition quickly.
2. Description of the Related Art
In an internal combustion engine, preignition is a phenomenon in which residual heat possessed by a spark plug or a deposit in a cylinder creates a hot spot causing the air-fuel mixture to be spontaneously ignited during the compression stroke. Preignition causes not only a sharp decrease in the engine output or a change in the engine revolution but also, in extreme cases, damage to the internal combustion engine.
Therefore, there has been proposed a device for controlling an internal combustion engine which so determines that preignition has occurred when vibration larger than a predetermined level is detected before ignition in each cylinder, and cuts off the supply of fuel to the cylinder (see Unexamined Japanese Utility Model Publication (Kokai) No. 1-88042).
According to the above-mentioned device for controlling the internal combustion engine which detects the occurrence of preignition when vibration larger than a predetermined level is detected before the ignition, however, it is only after the preignition has proceeded to a considerable degree that the occurrence of preignition can be detected. By that time, it is no longer possible to avoid the probability of causing damage to the internal combustion engine and, besides, drivability is deteriorated due to cut-off of the fuel supply.